Fibers for consumer use and the constituents that make up those fibers, especially fibers and polymers that are incorporated into carpet products and fabrics/textiles, are constantly being evaluated for improvements with respect to the durability and strength. These improvements may relate to tensile strength and tensile properties, quality, durability. Fibers and fiber products are also being evaluated to determine if there are more efficient and cost-effective manufacturing processes and equipment.
Most polymer processing does not only shape polymer into the desired shape (eg: injection molding, film blowing, fiber spinning, etc.). The processing is usually designed to impart desirable properties to the finished article by manipulation of the polymer molecules during the forming operation. For example, film blowing is carefully designed to combine the proper degree of stretching during the cooling of the molten polymer. This stretching orients the polymers improving strength and toughness of the film. Fiber is drawn during the cooling stage to control the degree of crystallization in the finished yarn.
Simple melt processing of polymers has distinct limitations. Since the manipulation is carried out during the brief period of solidification, practical limitations exist on how much molecular orientation can be achieved and/or how much shape manipulation can realized before the polymer is too cool to accomplish either. To be processed, the polymer must melt, the molecules freed-up for orientation, but still be sufficiently viscous and cohesive to hold together in the processing (a concept commonly called “melt strength”). These requirements restrict the molecular weights of the polymer that can be employed.
To overcome the restrictions imposed by the brief period of solidification, the polymer can be alternatively treated with a solvent material to produce a physical state that is in ways similar to the period during solidification. The polymer can be processed during that period to impart properties and/or shapes not achievable from simple melt processing. Gel-spun polyethylene has been exploited for the production of ballistic fibers.
Despite the obvious processing benefits of solvent gellation, very few examples have been explored. While the concept can be straightforward, successful implementation can be more difficult. The solvent-polymer system must achieve a gel that has sufficient solvation of the polymer molecules to manipulate the microstructure while still having sufficient melt strength to be processed into useful shapes. Once the article has been made, the solvent is typically removed to bring the polymer to its full properties. Solvent removal and recovery presents numerous cost and equipment issues to extract and dry the polymer and then recycle or otherwise dispose of the spent solvents. Industrial hygiene and environmental issues further complicate implementation. It is understandable why this technology has been limited to very high value materials like ballistic fibers.
For example, there is no recognized solvent system for gel processing of polyamide 6 (also known as nylon-6 or poly-caprolactam). Standard solvents like formic acid yield a solution far too low in viscosity for processing. Further, if nylon-6 gel processed products are to compete in applications other than high-cost ballistic fibers, a suitable solvent that is low-cost to implement is required. One candidate for gel processing nylon-6 products that has not been investigated is the epsilon-caprolactam monomer used to make nylon-6.
Caprolactam and nylon compounds have each been individually polymerized with the same compounds (i.e. caprolactam/caprolactam or nylon/nylon) by anionic polymerization in a twin-screw extruder and then tested for residual monomer content and therma/mechanical properties. In both methods, however, the polymerization process used was not designed to and did not result in a pre-fiber gel composition that could be processed by any means (including extrusion), and there was no indication that the mechanical strength or thermal strength was improved by the single monomer/twin-screw extruder mixing process. (see Antec '93 Conference Proceedings, New Orleans, 9-13 May 1993, Vol. 1, p. 470-473; and Antec '94 Conference Proceedings, San Francisco, Calif., 1-5 May 1994, Vol. 1, p. 116-22)
It was also known that caprolactam formed solutions with nylon-6, but these solutions took the form of residual, unpolymerized monomer found in the polymer. Typically nylon-6 resin is leached to remove this residual monomer. Deliberate addition of caprolactam for gel processing has not been previously considered. Gel processing and gel compositions are important because polymer parts have a practical limit in cross-section size due to the difficulty in forming such large cross-sections via melt processing. From a practical perspective, machines (extruders) to form polymer into shapes and sizes routinely available in metal simply do not exist. The other real limitation is that as melted polymer cools, significant shrinkage occurs. One can often find puckers on molded parts from shrink. To an extent, shrink can be compensated with clever mold design and tuning the molding process.
Therefore, it would be desirable to produce a pre-fiber gel material and/or composition that a) has sufficient viscosity and suitable cohesiveness such that it can be spun into a fiber or yarn, b) can be processed by any processing method, including extrusion, and c) can be incorporated into the production of a fiber, a fiber product, a yarn product and/or a carpet product. It would also be desirable to produce the pre-fiber gel material and/or composition at temperatures that are at or below normal processing temperatures for a polymer or monomer-based fiber product.